1. Field of the Invention
The present invention relates generally to a protective circuit for a transistor amplifier, and more particularly to a protective circuit for a transistor amplifier which is effective when used for protecting the amplifier against a decrease in a load impedance some predetermined critical point.
2. Description of the Prior Art
In a conventional protective circuit for a transistor amplifier, a protective line for the characteristic between the collector-emitter voltage V.sub.CE and the collector current I.sub.C of a transistor to be protected can be indicated by a line 1 shown in FIG. 1. In the case where a load is a pure resistance or a load similar to a pure resistance, for the protective characteristic, a load line such as indicated by a line 2 in FIG. 1, allows normal protective operation to be carried out. However, in the case where the load includes a reactance component, such as a condenser speaker, the load line becomes an ellipse 3 as shown in FIG. 1 and thus the load line of the ellipse 3 intersects with the protective line 1 with the result that the protective operation is also carried out for other than overload conditions. In such a case, since even if the load line becomes an ellipse 3, the ellipse 3 exceeds the tolerable collector loss of a transistor only for an instant and the transistor is not damaged in practice. In other words, in the case of the impedance load, the transistor is not destroyed even if the load line 3 becomes an ellipse which crosses the protective line 1, but the protective circuit operates which means the protective circuit operates when it should not.